1. Field of the Invention
The present invention relates to a color image processing system for processing a black and white image. More particularly, the present invention relates to a color image processing apparatus used in a color electrophotographic device, a color facsimile, a word processor, a personal computer, a printer, etc. The present invention also relates to an image forming apparatus for selectively providing a color image and a monochromatic image.
2. Description of the Related Art
Recently, there have been requirements of copying machines which can perform copying processing of both a black-and-white image and a color image in accordance with coloring of documents. To enable this color copying processing, many color copying machines are provided with a developing container having a black developer and a plurality of developing containers for respectively storing yellow, magenta and cyan developers as shown in Japanese Patent Publication (KOKOKU) No. 59-26954, Japanese Patent Application Laying Open (KOKAI) No. 62-180379, etc.
In the above color copying machines, an image is made in a black-and-white copying mode by using only the black developing container. In a color copying mode, respective decomposed images are overlapped with each other by using the respective yellow, magenta and cyan developing containers and the black developing container used in the above black-and-white copying mode in accordance with necessity, thereby performing the color copying processing.
In general, with respect to the quality of the black-and-white image in the black-and-white copying mode, it is most preferable to provide an image having less luster and no gloss in consideration of fatigue of eyes or to make the image easy to read. Therefore, the copying machines use the black developer adjusted such that the glossiness of the image is restrained as much as possible after a fixing processing in a combination of the fixing and developing processings. Namely, the copying machines use a black developer having a high melting temperature.
In contrast to this, with respect to the quality of the color image in the color copying mode, the image is bright and has a good appearance when the glossiness of the image is higher. Therefore, it is preferable to set the glossiness of the image to be high as much as possible after the fixing processing thereof. Accordingly, the black developer used in the color image processing has a low melting temperature.
To satisfy the above two conditions with respect to the black developer by a single developer, the melting temperature of the black developer is adjusted to provide an intermediate image quality with respect to the black-and-white image and the color image in the general technique as shown in Japanese Patent Application Laying Open (KOKAI) No. 62-180379.
In the Japanese Patent Publication (KOKOKU) No. 59-26954, a single black developer is commonly used to process both the black-and-white image and the color image. In this publication, a means for selectively moving a developing container is disposed to develop each color image in a single developing position so as to perform a so-called revolver development. The developing container of the black developer is separated from the other three developing containers to reduce a time for switching the respective developing containers in the revolver development. Thus, the developing container of the black developer is set in a developing position different from the developing positions of the developing containers of the other three developers.
In another proposed general technique of the copying machine, a fixing means is constructed by a plurality of fixing rollers. A surface of each of the fixing rollers is coated with e.g., Teflon-based resin, silicon rubber, etc. The glossiness of the image is restrained by the fixing rollers coated with the Teflon-based resin through the fixing processing of the image, which is suitable for the black-and-white copy image. The image is glossy by the fixing rollers coated with the silicon rubber through the fixing processing of the image, which is suitable for the color copy image. Such rollers coated with the Teflon-based resin, the silicon rubber, etc. are selectively used to perform a copying processing suitable for the black-and-white image or the color image.
However, the respective processed qualities of the black-and-white image and the color image are not completely satisfied in the above technique in which the melting temperature of the black developer is adjusted to obtain the intermediate image quality with respect to the black-and-white image and the color image. In other words, when the above black developer is used, the glossiness of the black-and-white image cannot be restrained to a minimum value and it is impossible to sufficiently prevent the fatigue of eyes and secure the image easy to read. In the case of the color image, it is impossible to increase the glossiness of the image until the image is bright and has a good appearance.
In the arrangement in which the developing container of the black developer is separated from the developing containers of the other colors, the developer of the separated black developing container is also used in the color image processing, it is impossible to perform the yellow, magenta, cyan and black developing processings in the same position of a photosensitive body in a series of color image processings. Therefore, all the developing conditions with respect to the respective colors is not constant at any time so that a control operation for switching the respective developing containers, etc. are complicated and there is a fear that a shift in color image is caused.
In the above technique, the image processing suitable for the black-and-white image and the color image is performed by selectively using the fixing rollers having surfaces coated with the Teflon-based resin or the silicon rubber. In such a technique, it is possible to sufficiently solve the above disadvantages when the entire image to be fixed is constructed by the black-and-white copy image or the color copy image composed of a picture, etc. However, for example, the copying machine in this technique cannot cope with an original having a character portion and a color picture portion mixed with each other so that it is impossible to secure the satisfied quality of the image as in the above-mentioned case.
In other word, Recently, an image forming apparatus such as a copying machine, a facsimile, a printer, etc. has been widely used in an office, etc. In particular, there are increasing requirements for a copying machine for providing a monochromatic copy and a full color copy in according with coloring of documents.
Therefore, the image forming apparatus is provided with a plurality of developing containers for respectively storing yellow, magenta, cyan and black developers. At the time of the monochromatic copy, the black developing container is operated to obtain a monochromatic image. At the time of the color copy, the yellow, magenta, cyan and black developing containers are operated such that yellow, magenta and cyan developed images are respectively overlapped and transferred onto the same transfer material so as to obtain a color image. In a digital copying machine, yellow, magenta, cyan and black developed images are respectively overlapped and transferred onto the same transfer material.
With respect to the quality of a copy image obtained by a monochromatic copying machine for business, it is preferable to provide an image having less luster and no gloss in consideration of fatigue of eyes or the image easy to read. To satisfy such requirements, it is desirable to use black toner for reducing the glossiness of a fixed image as much as possible.
In contrast to this, with respect to the quality of a full color copy image, the image is bright and has a good appearance when the glossiness of the image is higher. Therefore, it is desirable to use toner for increasing the glossiness of a fixed image to a certain extent.
Therefore, for example, as shown in Japanese Patent Application Laying Open (KOKAI) No. 62-180379, a melting temperature of the black toner is set to be higher than that of color toner so as to provide an intermediate glossiness for the overlapped images. However, in this structure, it is difficult to respectively satisfy qualities of both the monochromatic and color images.
As mentioned above, in the case of the monochromatic copy, it is desirable to provide an image having less luster and characters and diagrams are mainly drawn in an original, etc. Accordingly, it is desirable to provide a sharp image clearly showing edges. Such an image is called an image having an edge effect in the following description. In the case of the full color copy, an image having a large area and an intermediate color tone is copied in many cases so that it is desirable to provide a smooth and soft image. Such an image is an image provided by restraining the edge effect.
Further, in the digital copying machine, there is no problem with respect to an image having characters and diagrams since the image is formed by dots. However, when the image has a large area, image quality is easily reduced by dispersions in shape and position of dots since this image is formed by gathering these dots. In such a case, it is preferable to use a developing system for restraining the edge effect.
When the monochromatic image is formed, an original is mainly constructed by characters and diagrams so that it is desirable to provide an image having less luster in consideration of fatigue of eyes and the image easy to read. Further, frequency in formation of the monochromatic image is high in comparison with a color image so that it is necessary to form the monochromatic image at a high speed.
In contrast to this, when the color image is formed, it is desirable to provide a clear image having a high glossiness in consideration of a good appearance. Further, since the number of developing containers is increased, it is necessary to make the respective developing containers compact such that no image forming apparatus is large-sized.
U.S. Pat. No. 4,891,672 shows a monochromatic copying machine constructed such that images having single colors are respectively formed by developing devices. However, this patent does not shown a black developing container for a full color. If the black developing container for a full color is disposed in this monochromatic copying machine, the black developing container for a full color completely has the same construction as a black developing device and has no technical meaning.